This invention relates to improved scroll compressors wherein the pressure of fluid vented to a back pressure chamber is controlled and optimized.
Scroll compressors are becoming widely utilized in many air conditioning and refrigeration compressor applications. Some of the main benefits from scroll compressors are that they are relatively inexpensive and compact. However, scroll compressors do present challenges to achieve stable operation.
A known scroll compressor is illustrated in FIG. 1A. Scroll compressor 20 includes an orbiting scroll member 22 driven by a shaft 24. A fixed scroll member 26 has a helical scroll wrap 28 extending from a base plate and interfitting with a helical scroll wrap 27 extending from a base plate of orbiting scroll member 22. A discharge port 23 receives the compressed fluid. A back pressure chamber 29 is defined by a pair of seals 30 and 32 and a crank case 33. A vent hole 34 taps fluid from pressure chambers defined between the scroll wraps 27 and 28 to the back pressure chamber 29. The fluid tapped to back pressure chamber 29 is utilized to counteract a separating force created near the center axis of the orbiting scroll member 22 which tends to axially separate the orbiting and fixed scroll members 22 and 26. The force developed in the back pressure chamber 29 opposes this separating force, and maintains the orbiting scroll member 22 biased toward the fixed scroll member 26.
There are some deficiencies in this standard type of scroll compressor. In particular, the vent hole 34 is generally open to the pressure chambers defined between the scroll wraps 27 and 28 through the majority of the orbiting cycle of the orbiting scroll wrap 22. Thus, vent hole 34 communicates varying and pulsating pressures to back pressure chamber 29.
As shown in FIG. 1B, in any one pressure chamber, the pressure developed between the scroll wraps 27 and 28 varies during the operating cycle. The pressure increases from a low or suction pressure 41 to a high or discharge pressure 42. An intermediate pressure ramp 43 extends from the suction pressure 41 to the high pressure 42. The prior art vent hole 34 is typically exposed to intermediate pressure along a portion of ramp 43 and a portion of the high pressure 42. This period of exposure is illustrated by envelope region 47. Occasionally, the fixed scroll wrap 28 passes over hole 34 closing it momentarily. This closure is typically incidental and for a limited time. Thus, during the operational cycle of the scroll compressor, the pressure in the back pressure chamber 29 pulsates and may vary dramatically. This becomes particularly acute in high pressure ratio scroll compressor applications. That is, if the pressure ratio between low pressure 41 and discharge pressure 42 is relatively great, then the amount of pressure pulsation increases dramatically. Scroll compressors are now being considered for high pressure ratio applications. Thus, it can be expected that a good deal of pulsation would occur in the back pressure chamber 29 with the prior art venting.
Pulsation in the back pressure chamber has been found to result in back pressure chamber seal failure, and unstable operation. The pulsation results in a varying back pressure force to oppose the separating force between the orbiting and fixed scroll members. The varying force may not always successfully resist the separating force, particularly when the back chamber pressure is at a low point of the pulsation.
Another problem with the prior art is that pulsating pressures result in a relatively high amount of pumping losses from the pressurized fluid moving back and forth from the pressure chambers to the back pressure chamber. This pressure loss can be on the order of a few percentage points of the overall efficiency of the compressor, and thus is undesirable.
It is generally desirable to have a higher back pressure force resisting the separating force. However, it is also desirable to have some intermediate pressure in the back pressure chamber. Thus, locating the vent hole 34 only near the center of the scroll member such that it sees only relatively high discharge pressure may not always be fully desirable.
Other complications with regard to scroll compressors are found in particular applications. In some applications, a valve may be placed on the discharge port 23. The valve is selectively opened and closed in response to a discharge pressure 44 that is increased dramatically above an uppermost point 45 of the intermediate pressure ramp 43. When this occurs, pressures along the intermediate pressure ramp that are closer to the lower pressure range become particularly undesirable for use in back pressure chamber 29.
In other applications, point 45 may actually be higher than the discharge pressure 46. In these applications, eliminating the intermediate pressure altogether would be undesirable, as there are portions near the point 45 which are actually the highest operational pressures for the particular compressor application.
Thus, the problem of achieving optimum back pressure is not easily solved with the prior art vent hole.
The present invention overcomes the challenges in the prior art by developing a scroll compressor wherein the vent hole is only uncovered for a small portion of the operational cycle of the scroll compressor. The vent hole is effectively closed over the majority of the operational cycle of the scroll compressor. With this invention, a designer can ensure the vent hole is exposed to an optimum selection of intermediate and discharge pressures, which is communicated to, and maintained in, the back pressure chamber. Pressure pulsations are also reduced. In addition, with the reduction of the pulsation, the pumping losses found in the prior art are also reduced dramatically.
In a disclosed embodiment of this invention, the tapping or venting system is configured such that it selectively vents the fluid to the back pressure chamber from the pressure chambers at an intermediate pressure over a small portion of the cycle, and then vents the fluid at the discharge pressure over a separate small portion of the cycle. The vent hole is preferably closed between the tapping of the intermediate pressure portion and the discharge pressure portion. In this way, the system is able to achieve beneficial results by carefully selecting a desirable location and duration for tapping intermediate pressure and a desirable location and duration for tapping discharge pressure.
In one embodiment of this invention, the vent hole extends through the tip of the scroll wrap of the orbiting scroll. The hole is closed or abuts an end face of the base of the fixed scroll for the majority of its operational cycle. However, for a relatively small portion of its cycle it is exposed to an intermediate pressure. It is then again closed for a period of time, and then exposed to a discharge pressure for a small portion of its cycle.
In a preferred embodiment, grooves are formed in the base plate of the fixed scroll to tap the discharge and intermediate pressure to a location where they are periodically communicated to the vent hole in the orbiting scroll wrap as the orbiting scroll wrap moves relative to the fixed scroll wrap.
In other embodiments of this invention, the vent holes are formed through the base plate of the orbiting or fixed scrolls. The scroll wrap of the other scroll member is positioned over the vent hole for the majority of the operational cycle of the scroll compressor. However, the vent hole is opened for a small portion of the cycle of the scroll compressor where it would be exposed to an intermediate pressure, and also for a small portion where it would be exposed to a discharge pressure. In a most preferred embodiment of this aspect of the invention, there are actually two vent holes utilized, both being in communication with the back pressure chamber and with one being periodically communicated to intermediate pressure and the other being periodically communicated to discharge pressure.
These and other features of the present invention will be best understood from the following specification and drawings, of which the following is a brief description.